I would not touch either a Mac or a Dell. Macs are just too expensive, and they become obsolete just as fast as any other notebook. The Mac software is expensive too, and the software selection is more limited. As for Dell, the components are cheap. Same for HP, Compaq and Toshiba.
I personally prefer Sony Vaios, but the Lenovos still have the best keyboard in the business and make a very nice smaller machine. It has a 13.3 inch screen and weighs under 4 lbs. (It's a true laptop, not one of those el cheapo netbooks.) The ASUS UL30A is another nice smaller laptop at a good price.
If you want something larger it's hard to beat the Sony Vaio FW series with that very nice 16.4 inch screen. I have two vaio's, an FW and a Z, and I love both of them.